Coil shielding method for selective attenuation of an electromagnetic energy field component

ABSTRACT

A shield apparatus for use in conjunction with a well tool to selectively attenuate one or more electromagnetic energy field components as the components interact with the shield. The shield composed of a flexible strip or conductive body and comprising at least one sloped slot or sloped conductive element therein. The shield being adapted to surround an antenna mounted on a well tool. A method for rotating the axis of the magnetic dipole of a transmitter or receiver coil. A method for winding and shielding an electric coil such that the resultant coil emits or receives selected electromagnetic energy field components.

CROSS-REFERENCES

The present application is a divisional of U.S. patent application Ser.No. 09/452,660, filed Dec. 1, 1999, now U.S. Pat. No. 6,351,127.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of well logging tools of the typewherein electromagnetic (“EM”) energy is used for measuringcharacteristics of formations surrounding a borehole. More particularly,this invention relates to an improved antenna coil shield for use insuch tools to provide selective attenuation of the EM waves emitted orreceived by the antenna.

2. Description of Related Art

Induction and propagation well tools have been employed in loggingoperations for many years to measure the properties of subsurfaceformations surrounding an earth borehole. In conventional loggingtechniques, a number of antennae or coils are mounted on a well tool. Analternating current energizes one or more transmitter coils to emit EMenergy into the formations. The emitted energy propagates through theformations or induces currents in the formations surrounding theborehole. The EM energy or currents are detected and measured by one ormore receiver coils on the tool. The measured EM signals are processedto determine the electrical properties, such as permittivity orconductivity, of the formations.

If the transmitter and receiver coils on these tools were perfectlyconfigured and balanced in a theoretically ideal system, the EM energyemitted by the coils would propagate in a mode known as a transverseelectric (“TE”) mode, of the type generated by an ideal verticalmagnetic dipole in an azimuthally symmetric media. However, under actualoperating conditions, there are various factors that give rise to thegeneration of significant undesired EM field components. One approach toalleviating this problem is with the use of antenna shields to reducethe transmission and/or reception of spurious and unwanted EM fieldcomponents. These shields are typically used in conjunction with eachcoil on the tool.

U.S. Pat. Nos. 4,536,714 and 4,949,045 (both assigned to the assignee ofthe present disclosure) disclose conventional antenna shields employedin these well tools to provide mechanical protection for the coils andto guarantee the passage of desired EM field components. As shown inFIG. 1a, these shields 10 are in the form of a metal cylinder that hasslots 12 in the axial direction. The slot 12 pattern allows theazimuthal electric field (Eφ) component of the EM energy to pass, butprevents radial (Er) and axial (Ez) electric field components frompassing through the shield, either from within (in the case of atransmitter) or from without (in the case of a receiver). An alternativeviewpoint is to represent each axial slot 12 as an axial magneticdipole, as shown in FIG. 1b. These magnetic dipoles are sensitive toaxial magnetic fields (Bz), but they are not sensitive to azimuthalmagnetic (Bφ) fields. The shielded coils are thus rendered insensitiveto parasitic transverse magnetic (“TM”) EM fields associated withborehole modes, and which have radial (Er) and axial (Ez) electricfields and azimuthal magnetic fields (Bφ).

Recent publications in the field of well logging have described theimplementation of tools with triaxial coils. Such coil configurationsinvolve three coils with magnetic moments that are not co-planar. U.S.Pats. Nos. 5,508,616, 5,115,198, 5,757,191 and PCT Application WO98/00733, Bear et al., describe logging tools employing such coilconfigurations. Common to these apparatus and techniques, however, isthe need to manipulate the antenna coil itself. None of thesedisclosures address the implementation of antenna shields as alternativemeans to achieve selective EM energy attenuation.

It is desirable to rotate the axis of the magnetic dipole of atransmitter or receiver coil without having to tilt the axis of the coilin relation to the tool axis. The benefits of such a technique includereductions in manufacturing and re-tooling costs, as well as shorterproduction times. Still further, it is desired to implement a shieldapparatus that can be used in conjunction with tilted and non-tiltedcoils to rotate the axis of the magnetic dipole.

SUMMARY OF THE INVENTION

A shield apparatus adapted for use in conjunction with a well tool isprovided to selectively attenuate one or more electromagnetic energyfield components as the components interact with the shield.

In a first aspect of the invention, a shield containing a sloped slotpattern is provided to surround a coil. The shield attenuates selectedelectromagnetic energy field components as they interact with the shieldto pass the desired components and restrict unwanted components.

In a second aspect of the invention, a strip is provided to surround acoil. The strip contains conductive elements configured in a slopedpattern. The strip thereby attenuates selected electromagnetic energyfield components as they interact with the shield to pass the desiredcomponents and restrict unwanted components.

In a third aspect of the invention, a method for rotating the axis ofthe magnetic dipole of a transmitter or receiver coil is provided.

In a fourth aspect of the invention, a method for winding and shieldingan electric coil is provided. The resulting coil emits or receivesselected electromagnetic energy field components.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to thedrawings in which:

FIG 1 a is a schematic diagram of a conventional cylindrical shield withaxial slots. Directed arrows are representative of the interactionbetween the shield and the electric field components of incidentelectromagnetic energy.

FIG 1 b is a schematic diagram of a conventional cylindrical shield withaxial slots. Directed arrows are representative of the interactionbetween the shield and the magnetic field components of incidentelectromagnetic energy.

FIG. 2 is a schematic diagram of a coil wound at an angle θ to thelongitudinal axis of well tool. Also depicted is a view of the tiltedcoil as projected onto a two-dimensional surface.

FIG. 3 is a schematic diagram of a sloped slot pattern superimposed ontoa tilted coil and projected onto a two-dimensional surface. The slotsare maintained perpendicular to the coil winding(s).

FIG. 4 is a schematic diagram of a sloped slot pattern superimposed ontoa non-tilted (axial) coil and projected onto a two-dimensional surface.

FIG. 5 is a schematic diagram of the sloped slot pattern of FIG. 4 withthe slots maintained centered over the coil winding(s).

FIG. 6 is a perspective view of a cylindrical shield in accord with thepresent invention.

FIG. 7a is a schematic diagram of a cylindrical shield in accord withthe invention. Dashed arrows represent the axial magnetic dipole andtransverse magnetic dipole components associated with the slot patternof the shield.

FIG. 7b is an overhead cross-section of a tool with the shield of FIG.7a as seen along line A—A when the tool is in a borehole.

FIG. 8 is a schematic diagram of a shield composed of a strip in accordwith the present invention. The strip is shown projected onto atwo-dimensional surface.

FIG. 9 is a schematic diagram representative of a set of transversemagnetic moments oriented about a longitudinal axis.

FIG. 10 is an unwrapped view of a shield composed of a strip containingmultiple conductive elements in accord with the present invention.

FIG. 11 is a diagram of the shield of FIG. 10 superimposed over thewindings of a tilted coil in accord with the present invention.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In the interest of clarity, not all feats of actual implementation aredescribed in this specification. It will be appreciated that althoughthe development of any such actual implementation might be complex andtime-consuming, it would nevertheless be a routine undertaking for thoseof ordinary skill in the art having the benefit of this disclosure.

As discussed above, conventional shields used in well tools universallyhave slots that are aligned along the longitudinal axis of the tool. Theorientation of the slots is perpendicular to the electric fieldgenerated by the coil within or the field that is to be detected by thereceiver. If the incident field has an unwanted component of theelectric field that lies along the slot, then currents will flow in themetal to cancel that field and only the normal component will remain.For conventional induction or propagation tools, the desired electricfield is azimuthal, and longitudinal slots allow that field to pass. Ifthe coil was wound at an angle θ to the axis of the tool, then thedesired electric field is no longer azimuthal, but rather has bothazimuthal and longitudinal components that vary as a function of theazimuthal position.

FIG. 2 illustrates a coil 14 wound at an angle θ to the longitudinalaxis (represented by dashed lines) of the tool and having radius α.Projecting the coil 14 onto a two-dimensional surface as shown, theheight of the coil 14 is described by a sinusoidal function of theazimuthal angle around the tool φ:

ƒ(φ)=α tan θ cos φ.  (1)

An actual coil would probably have multiple windings, described byequation (1), but with an additional term pφ, where p is the pitch.Effective shields for such coil configurations should preserve both themechanical advantages and the EM advantages offered by conventionalshields.

Sloped Slot Pattern

A shield to let pass the desired EM field components, and attenuate theundesired ones, should have at least one sloped slot that is sloped atan angle θ with respect to the tool axis. A sloped slot pattern for atilted coil 14, projected onto a two-dimensional surface, is shown inFIG. 3. The slots 12′ are perpendicular to the coil 14 at theintersection of the slot 12′and coil 14. This allows the electric fieldcomponent that is parallel to the coil 14 to pass through the shieldwith minimal attenuation. This electric field will have azimuthal andaxial components, but no radial component. The slope of the slot 12′ isgiven by

 1/(α tan θ sinφ).  (2)

Alternatively, one can represent the slots 12′ as point magnetic dipoleson the surface of a conducting cylinder (not shown). The location ofeach magnetic dipole is given by equation (1), and their orientation isgiven by equation (2). Each individual magnetic dipole has an axialcomponent and a smaller azimuthal component.

While the above discussion has assumed that the coil under the shield istilted at an angle θ with respect to the tool axis, the shields 10 ofthe present invention can also be used with an axial coil 14. With thisconfiguration, the axis of the magnetic dipole of the coil can beselectively rotated. FIG. 4 illustrates a sloped slot 12′ patternsuperimposed onto an axial coil 14 and projected onto a two-dimensionalsurface. This configuration will deviate from the configuration of FIG.3.

As shown in FIG. 4, the slots 12′ are no longer perpendicular to thecoil 14 windings. This may affect the relative strength of thetransverse magnetic dipole (“TMD”) component to the axial magneticdipole component. One approach to minimize these effects would be tomaintain the slots 12′ centered over the coil 14, as shown in FIG. 5.FIG. 5 also shows a sloped slot 12′ pattern superimposed onto an axialcoil 14 and projected onto a two-dimensional surface. Although the coil14 in FIG. 4 is shown comprising multiple windings, it will beunderstood by those skilled in the art that the shields of the presentinvention are effective with coils 14 composed of one or more windings.

While FIGS. 3-5 show straight slots 12′, in general the slots 12′ arecurved in order to maintain the direction of the slots 12′ perpendicularto the directions of the winding(s) or to keep them perpendicular to thedesired direction of the electric field that is to pass through theshield without attenuation.

Surrounding an axial coil 14 with a shield of the present invention willproduce transverse magnetic fields. Only the component of the electricfield perpendicular to the slot 12′ will pass through the shield; thecomponents parallel to the slot 12′ will be attenuated. The electricfield that passes through the slots 12′ is in the direction that wouldresult from a true tilted coil. Basically, the shield functions as apolarizer that passes components of the EM field corresponding to amagnetic dipole oriented at an angle tilted relative to the tool axis.

FIG. 6 shows an embodiment of a shield 10 of the present invention. Oneembodiment of the shield 10 entails a hollow body 16 formed of aconductive material, typically metal. The shield 10 has aperts 18 at itsends through which the tool body (not shown) passes. Typical well toolscontain an elongated metallic pipe as a central support means upon whichsensors, electronics, and other instrumentation are mounted. It will beunderstood that other support means, such as coiled tubing ornon-metallic pipes, may be used to implement the present invention, asthe precise type of support means is immaterial here. The hollow body 16may be open-ended or closed-ended. The body 16 is generally formed inthe shape of a surface of revolution. A cylinder is preferred, althoughother shapes, such as an ellipsoid of revolution may be employed.Preferably, a shield 10 will independently surround each coil on thewell tool. The shield 10 may be mounted on the well tool in a suitablemanner as know in the art.

The EM radiation pattern around a logging tool may be affected by thetool itself, so optimum shield 10 operation may require fine tuning theexact slot 12′ pattern. Modeling shows that borehole eccentricity canhave a large deleterious effect on a measurement using TMDs. EccenteredTMDs can couple directly into TM borehole modes via the TM mode'sazimuthal magnetic field (Bφ). Since a tilted coil 14 can be representedas a vector sum of an axial magnetic dipole and a transverse magneticdipole, it will also be susceptible to large eccentricity effects.

However, the shield 10 configurations of the present invention willprovide some immunity to the TM mode, so the eccentricity effects may bereduced in severity. FIG. 7a shows the axial magnetic dipole componentB_(A) and the transverse magnetic dipole component B_(T) associated witheach slot 12′. As shown in FIG. 7a, the slot 12′ pattern provides forsome cancellation of opposing transverse magnetic dipole componentsB_(T).

FIG. 7b is an overhead view of a tool with the shield of FIG. 7a as seenalong line A—A when the tool is in a borehole. As shown in FIG. 7b, theTM mode's azimuthal magnetic field (Bφ) may couple to the transversemagnetic dipole components B_(T) of the slots 12′. The TM mode's radialelectric field (Er) will not penetrate the shield 10, so the coil 14will not sense this.

The transverse magnetic dipoles vary with azimuth φ as sinφ. The TMmode's magnetic field may be written as

B _(φ)(φ)=B ₀ +B ₁sinφ+B ₂sin2φ+  (3)

The field B₀ will not be detected by the coil 14 because B₀ is an evenfunction of φ, while the transverse magnetic dipoles are an odd functionof φ. The same is true for B₂ sin2φ. However, B₁ sinφ is an odd functionof φ, so it will be detected by the coil 14. Assuming that theconductivity of the tool is many orders of magnitude larger than theborehole fluid or the formation, the azimuthal magnetic field (Bφ) willnot vary much with azimuth φ. Hence, B₀>>B₁, B₂, so that the TM couplingto the slots 12′ will be very small on average.

Modifications may be made to the shield 10 or the coil 14 to alter theazimuthal amplitude of incident EM energy or the angle of rotation ofthe magnetic dipole. Multiple shields 10 may be overlaid coaxiallyaround a coil 14. Combinations of sloped and axial slots of varyinglength, width, thickness, orientation, symmetry, density, or spacing maybe formed on a shield 10. The sloped slots 12′ may have equal or variedslope angles. The slots 12′ may be partially or entirely filled withsome sort of lossy (i.e., conductive) material. A conductive element,such as a metallic strap or wire, may be connected between the sides ofa slot 12′ to partially short out the slot 12′.

A shield 10 of the invention may also be formed comprising two halves orseveral sections configured to form a surface of revolution whencombined (not shown). Such a configuration may further comprise onesection or one half of the shield 10 being electrically isolated fromthe other half or other sections. The spacing between the coil and itssupport means or the spacing between the coil and the shield 10 may alsobe varied. It will be appreciated by those skilled in the art having thebenefit of this disclosure that other modifications may be employed toincrease the efficiency of the shield 10.

Strip Shield

FIG. 8 illustrates another embodiment of the present invention. A shieldmay be implemented in the form of a strip 20, also referred to as a flexcircuit. The strip 20 is shown projected onto a two-dimensional surfacefor clarity of illustration. An effective strip 20 may be formed of anysuitable non-conductive material that can be adapted to coaxiallysurround a coil. The strip 20 is preferably flexible, but it may also beformed of a rigid material. The strip 20 contains at least oneconductive element 22, preferably a multitude of elements 22. Theconductive elements 22 may be formed of fine strips of copper or othersuitable conductive materials.

As described above, a shield incorporating sloped slots may be used torotate the magnetic moment of a coil 14. Thus, the conductive elements22 are disposed in the strip 20 such that each element 22 is sloped atan angle with respect to the tool axis when the strip is mounted on thetool to surround a coil. Since the strip 20 is non-conductive (unlikethe shield embodiments described above), the elements 22 must also beconfigured to form a loop around the coil when the strip surrounds thecoil. The loop provides the path in which currents can flow around thecoil in order to rotate the axis of the magnetic dipole. The strip 20provides selective attenuation of the EM energy emitted or received by acoil when a complete loop is formed around the coil by the conductiveelement 22.

A switchable connection is provided in the strip 20 to selectively openor close the loops formed by the conductive elements 22, as illustratedin FIG. 8. This connection may be a series of connections or only oneconnection. The connection(s) may also be located at any suitable pointin the circuit. When the connection is closed, the element 22 acts torotate the magnetic dipole of the coil. When it is open, it has noeffect. One form of a switchable connection utilizes a MosFET switch toopen or close the current path around the coil. Other suitable means maybe utilized to form the switchable connection(s) as known in the art.The strip 20 may also comprise additional switching means (not shown) toprovide an electrical short with the mandrel of a well tool if needed.

The modifications described above may also be made to the strip 20 orthe coil to alter the azimuthal amplitude of incident EM energy or theangle of rotation of the magnetic dipole. Multiple layers of conductiveelements 22 having different directions of magnetic dipole moments mayalso be disposed on the strip 20. This would allow the use of a singleaxial coil 14 as a transmitter or receiver and by closing the switchableconnection(s) on the strip 20, different rotations of the magneticmoment could be achieved. Alternatively, multiple strips 20 could beoverlaid coaxially to surround a coil.

Transverse Magnetic Dipoles

By altering the direction of the magnetic dipole, a coil can be used tomake formation measurements at multiple orientations. This sectiondescribes a method for winding and shielding a coil structure to producea set of TMDs in accord with the present invention.

By superimposing or overlaying three coils around a non-conductivesupport means and wrapping the coils with one or more strips 20, a setof transverse dipoles may be produced. FIG. 9 illustrates a set ofmagnetic moments directed along three orthogonal directions at an equalangle to the longitudinal axis of the tool. With this configuration, thethree coils and their corresponding strip(s) 20 can be turned on or offindependently. This allows for any one coil and polarizer pair to beengaged, while the other two sets are disengaged.

The construction of a coil and polarizer strip 20 for the simplest case(which would be just one coil and its corresponding polarizer) will nowbe described. The coil may be wound around a non-conductive supportmeans (such as an insulated tool mandrel) from any suitable conductivewire as known in the art. Referring to FIG. 2, to produce a magneticdipole at some angle Φ between 0° and 90°, the location of the center ofthe thread should follow

Z(φ)=−αtanΦcosφ+pφ,  (4)

where a is the radius of the support means, φ is the azimuthal angle,and p is the pitch. The wire is preferably wound closely packed so thatthe thread depth and width are on the order of the wire diameter d andα>> p ≧d.

The polarizer strip 20 is constructed so that the conductive elements 22are everywhere perpendicular to the current in the coil windings. FIG.10 shows an embodiment of a strip 20 containing conductive elements 22in accord with the invention. The conductive elements may be embedded,glued, or affixed to the strip in any suitable manner as known in theart. The functional form ƒ(φ′) of these conductive elements 22 isderived by $\begin{matrix}{{{f\left( \varphi^{\prime} \right)} = {\int{\frac{- 1}{\frac{z}{\varphi}}{\varphi^{\prime}}}}},} & (5)\end{matrix}$

where

dz/dφ=−α tan θ sin φ′,  (6)

evaluated at φ=φ′. Therefore, $\begin{matrix}{{{f(\varphi)} = {{\int\frac{1}{\beta \quad \sin \quad \varphi}} = {\frac{1}{2\beta}\ln \quad \left( \frac{1 + {\cos \quad \varphi}}{1 - {\cos \quad \varphi}} \right)}}},} & (7)\end{matrix}$

where β=α tanΦ.

In addition to providing selective attenuation of EM energy components,the polarizer strip 20 acts as a Faraday shield to reduce capacitivecoupling between coils, without attenuating the desired components ofthe magnetic field. The behavior as a Farady shield is comparable to thebehavior of conventional shields used on present generation induction orpropagation tools. FIG. 11 shows the strip 20 of FIG. 10 superimposedover the windings 24 of a tilted coil. As shown in FIG. 11, theconductive elements 22 are everywhere perpendicular to the coilwindings. Although FIG. 11 shows the superposition of a strip 20 over acoil 14, the same pattern applies to the superposition of a cylindricalshield 10 with sloped slots 12′ over a coil 14. The simplified coil andstrip 20 described above can be overlaid to create a set of basismagnetic dipoles. These can be used to construct a coil structure thatprovides selective three-dimensional measurement capability.

While the methods and apparatus of this invention have been described asspecific embodiments, it will be apparent to those skilled in the artthat variations may be applied to the structures and in the steps or inthe sequence of steps of the methods described herein without departingfrom the concept and scope of the invention. All such similar variationsapparent to those skilled in the art are deemed to be within thisconcept and scope of the invention as defined by the appended claims.

What is claimed is:
 1. A method for winding and shielding an electriccoil, whereby the coil emits or receives selected electromagnetic energyfield components, comprising: a) winding a first conductive wire arounda support means such that each turn of the windings is not perpendicularto the longitudinal axis of the support means; b) advancing the firstwire around the support means to form at least one layer of windings;and c) wrapping the windings with a flexible strip adapted to provideselective attenuation of the emitted or received electromagnetic energyfield components.
 2. The method of claim 1, wherein a second conductivewire is wound around the non-conductive support means such that theturns of the winding are neither perpendicular to the longitudinal axisof the support means nor parallel to the turns of the first wire, andadvancing the second wire around the support means to form at least onelayer of windings.
 3. The method of claim 2, further comprising wrappingthe windings of the second wire with a flexible strip adapted to provideselective attenuation of the emitted or received electromagnetic energy.4. The method of claim 3, wherein the flexible strip wrapped around thesecond wire includes at least one conductive element disposed therein.5. The method of claim 4, further comprising switching means connectedto at least one conductive element on the flexible strip wrapped aroundthe second wire.
 6. The method of claim 2, wherein a third conductivewire is wound around the non-conductive support means such that theturns of the winding are neither perpendicular to the longitudinal axisof the support means nor parallel to the turns of the first or secondwire, and advancing the third wire around the support means to form atleast one layer of windings.
 7. The method of claim 6, furthercomprising wrapping the windings of the third wire with a flexible stripadapted to provide selective attenuation of the emitted or receivedelectromagnetic energy.
 8. The method of claim 7, wherein the flexiblestrip wrapped around the third wire includes at least one conductiveelement disposed therein.
 9. The method of claim 8, further comprisingswitching means connected to at least one conductive element on theflexible strip wrapped around the third wire.
 10. The method of claim 1,wherein the flexible strip includes at least one conductive elementdisposed therein.
 11. The method of claim 10, further comprisingswitching means connected to at least one conductive element.